Piston Slap?

[Inka’d02]

Good Morning!

 

I wanted to throw this out to the group and see if anyone had any thoughts. I had the motor rebuilt in my 73 about 7 years ago and since then I have put about 20K on it. In the past year or so I have noticed that when I start the car up cold it would make an intermittent knock noise. It does not remain stay continuous, that is to say it will make the wrap/knock noise for the a few revolutions then go away and then come back sporadically. This seems to happen at idle rpm only and goes away when warmed up. This has become more and more pronounced to the point now where I can not ignore it and I feel like even when warmed up I can hear it mixed in the sounded of the normal rocker tick. I am planning to pull the motor over the winter to sort this out but wanted to get your guys thoughts as well and see if anyone has any conclusive tests that can be done. I will try to get a video posted this weekend.

 

Thanks!

Andy

Edited August 2, 2019 by Inka’d02

[2002Scoob]

Do you have a mechanic's stethoscope? 

 

That could go a long way to diagnosing the what, where, and why of your mystery noise. Start poking, prodding, and listening to where the noise could be coming from. 

 

Have you checked your oil-pressure in different scenarios? This seems to be a hot-topic lately. Maybe you're seeing low pressures, or perhaps just low oil levels?

 

Best of Luck,

-J

[Guy Cocquyt]

Could this be timing chain? Noisy at start and then quites down once it is lubricated with oil.

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[Sahara]

I’m a pathetic noob but let’s think this through a bit:

-piston slap comes from very wide clearances in the motor

-a recently rebuilt motor should not have such clearances 

 

My car can make what I imagine is a similar noise when she’s running to lean after cold start. A bit of extra choke clears it up. I’d check for incomplete or strange combustion due to mixture issues before going down the piston slap route. 

 

For the more experienced guys here, wouldn’t slap also manifest as increased vibration and reduced power throughout the rev range? 

 

[tzei]

When it was rebuild were conrod bushes replaced? If you end up digging this deep in the motor be sure to check those clearances. Timing chain components are good call too.

[Inka’d02]

Thanks, 

 

Chain, tensioner, and guide are new but please let me know what  you think might be the issue there.

Conrod bushings were done. Would that get quieter as it warms up?

Great oil pressure- 65 psi idle when cold. 40 at idle when hot.

Did use a stethoscope. Def somewhere in the short block but I will take a listen tot he timing chain cover area tonight. 

 

Thanks for the thoughts so far. 

 

[Hans]

1 hour ago, Sahara said:

I’m a pathetic noob but let’s think this through a bit:

-piston slap comes from very wide clearances in the motor

-a recently rebuilt motor should not have such clearances 

 

 

 

Depends on whether rebuilder recycled the pistons, and what teh resulting clearances were. Piston slap typically goes away (somewhat) as engine warms up.

[Sahara]

Just now, Hans said:

Depends on whether rebuilder recycled the pistons, and what teh resulting clearances were. Piston slap typically goes away (somewhat) as engine warms up.

Right. So on my car with 4 new pistons at 30k miles ago it’s probably not a concern? 

[TobyB]

Piston slap won't be intermittent, and will be most present at start- up, increasing with revs.

It won't go away when you rev the engine, as the pistons will be flopping around more.

 

I've heard this intermittent 'knock' on several engines, and it's been several things 

varying from small- end bushings to a worn cam to to an exhaust leak.  To a worn- out engine.

Try methodically with the stethoscope- if it doesn't localize, REALLY check the exhaust

gasket(s)!

 

hth,

 

t

 

[Dudeland]

I had a clickyclackity when started up and intermittent clicks. It was two things.

1) timing chain tensioner
2) bots for the oil pump pickup brace that hangs off the number two and big end bearing cover came loose and the bracket was vibrating.


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[Mikesmalaga72]

I`ve heard forged pistons, or cracked flywheel/flex plates could do that in some instances.

[JsnPpp]

50 minutes ago, Mikesmalaga72 said:

I`ve heard forged pistons, or cracked flywheel/flex plates could do that in some instances.

 

Agreed, my forged 10:1 piston car has had that sound during my entire 17 year ownership. It is brief but present. Previous owner acknowledged it as well and it hasn't changed over this timeframe. Different alloys expand at different rates, with initial cold clearance being greater than clearance during running operating temperatures. --> As the engine warms up, the forged pistons expand when they get warm. 

If it's a new sound or a changing sound, that could be indicative of a different issue, but does not necessarily indicate something requiring a full rebuild.. need to dig in a bit and see if you can pinpoint..  Great insight in this thread!

 

Jason

[Hans]

If the pistons were new, and the bore is correct, then I'k look elsewhere.

[Inka’d02]

They were a new set of max-sil pistons and cylinders were re-bored.

 

What about the timing chain tensioner can cause the knocking noise? I am really curious about this one.

 

I like the small end bushing as well but they were done, unless maybe one failed...

Edited August 2, 2019 by Inka’d02

[JerryC]

Wasn't an '02, but I've heard of a case where a too-thin crankshaft thrust bearing caused a knocking noise at startup.